The present invention relates to DC-DC converters and more particularly to a circuit to step-down output voltage to a lower voltage and power level.
Isolated DC-DC converters, whether part of a complete AC-DC system or just pure DC-DC system, often require several output voltages at different power levels. These output voltages are normally obtained through multiplication of a transformer's secondary windings for each output at higher power level, e.g., 30 W and up, while simple buck converters are often used for step-down stages to a lower voltage and lower power level, generally less than 30 W, from one of the output voltages.
FIG. 1 illustrates a DC-DC converter 10 having a primary stage 12 having a DC bus as input and an output connected to a primary coil LP of a transformer 14. The transformer includes first and second secondary coils LS1 and LS2 series connected at a node. The first and second secondary coils LS1 and LS2 are connected to first and second output rectification circuits 16 and 18. Each rectification circuit 16 and 18 has an Output 1 and 2, respectively including a capacitor Cout1 and Cout2, respectively.
The DC-DC converter 10 further includes a synch buck control circuit 20 for controlling a third switching stage 22 with Output 3. The switching stage 22 includes high- and low-side switches M1 and M2 connected in series at a switching node. The high-side switch M1 is further connected to the output of the output rectification circuit 18 and the low-side switch M2 is farther connected to ground. Additionally, a first terminal of an inductor L1 is connected to the switched node VS and its second terminal, in a local feed back loop, to the synch buck control circuit 20. Further, a capacitor Cout3 is connected between the second terminal of the inductor L1 and ground.